基于纳米盘棒耦合的多频段等离激元诱导透明研究

提出了基于银纳米棒和银纳米盘的多频段等离激元诱导透明(PIT)混合模型,通过时域有限差分法研究了模型的电磁特性.研究表明:由于银纳米盘(明模)和银纳米棒(暗模)的明模-暗模-暗模耦合,模型可以产生双频段的PIT效应.在双频段的基础之上,通过两个非对称的双频段PIT模型的叠加,形成暗模-暗模-明模-暗模-暗模耦合,可进一步实现四频段的PIT效应.同时,只要改变两种PIT模型中银纳米棒的长度以及银纳米棒和银纳米盘之间的距离,双频段PIT和四频段PIT窗口的谐振频率和透射振幅都会随之变化.最后研究了四频段PIT模型的传感效应,发现该模型随背景材料折射率变化的灵敏度(sensitivity)达到了326.2625 THz/RIU,优值系数(FOM)达到了26.4/RIU,性能优于其他同类型传感器,这为该模型在光存储、吸收、滤波和红外频段的传感器设计中的应用提供了理论参考.     

拉锥光纤中光传输模场的矢量分析

采用二维矢量时域有限差分法(FDTD),考虑了拉锥光纤芯层的折射率分布和光纤芯层直径等参数,详细分析了拉锥光纤的锥度、拉锥后纳米光纤半径等对拉锥光纤中光波模场分布和传输特性的影响。计算结果表明,单模光纤芯层的折射率分布函数对光纤中光传输模场分布影响较小;拉锥光纤锥度越大光波的能量损耗越大;当拉锥后纳米光纤半径逐渐变小时,纳米光纤的光传输模场中心光强先逐渐增强然后又逐渐减小。     

两模腔场谱间的量子干涉

研究了高Q腔中单个二能级原子与两模二项式光场依赖强度耦合相互作用系统的腔场谱，给出了弱初始场条件下的数值结果，讨论了两模光场之间的量子干涉对腔场谱结构的影响. 发现当两模光场的频率差Δ>g（g为原子与腔场间的耦合常数）时，两模光场间的干涉效应对谱结构没有影响，系统的腔肠谱只是两模腔肠谱的简单叠加；当Δ≤g时两模腔场谱间的干涉比较明显. 在强初始场条件下，量子干涉效应可忽略. 关键词： 腔场谱 量子干涉 两模二项式光场     

一种由椭圆形金属和介质纳米线组成的混合表面等离子体光波导的基模控制特性研究

本文研究了一种由三根并排放置的椭圆形金属-介质-金属纳米线构成的混合表面等离子体光波导所支持的电磁场基模的控制特性,中间是高折射率的介质纳米线,左右是两根对称放置的金属纳米线。研究结果表明,基模电磁场增强效应主要分布在三根纳米线形成的两个间隙区域,且对整个结构的几何参数有一定依赖性。因此,通过改变纳米线的几何尺寸、两根纳米线之间的间距以及介质的电磁参数,可以调整和控制这种波导所支持的基模的有效折射率、模式传输距离、归一化的模式面积和模式束缚因子等物理特性。基于这些有效的模式操控特性,这种混合型的表面等离子体光波导可以应用于高密度光子器件集成、纳米光子学和生物传感器等领域。     

基于耦合介质纳米线的深亚波长局域波导

提出了一种基于耦合介质纳米线的深亚波长局域波导,通过两根紧邻的高折射率介质纳米线的耦合,该波导可以将光场有效束缚在纳米线之间的低折射率纳米缝隙中. 计算模拟的结果表明,该波导的有效模场面积达到Λ²₀/200,比单根纳米线波导小一个数量级,这种深亚波长的模场束缚能力可以与表面等离激元混合波导相比拟. 计算模拟的结果还表明,纳米线可能带有的低折射率氧化膜、低折射率衬底的存在、以及纳米线间尺寸存在的一定差异对于该波导结构的实际应用都不会产生很大 关键词： 介质波导 亚波长局域 表面等离激元波导 纳米线     

单模光纤中偏振模色散的仿真模型

利用琼斯矩阵法研究了长单模光纤中偏振模色散的仿真模型.考虑到偏振模色散的随机性,该模型中单模光纤被看作是一系列短双折射光纤段的级联,相邻两段之间耦合角是随机的.研究结果表明,当短双折射光纤段等长时,偏振模色散呈现随波长周期性变化的特点;不符合实际情况.当短双折射光纤段不等长且服从高斯分布时,周期性逐渐消失;当其长度均方差为均值的20%,周期性完全消失.最后比较了偏振模色散的时域统计特性.取短双折射光纤段的长度服从高斯分布且均方差为均值的20%,偏振模色散的统计特性接近于实际分布.因此得出结论:为了正确估计偏振模色散的影响,在单模光纤的级联模型中,短双折射光纤段的长度应服从高斯分布,均方差为其均值的20%.     

具有二阶和三阶非线性一维光子晶体中的耦合模孤子

研究同时具有二阶和三阶非线性的一维光子晶体中的耦合孤子动力学.从Maxwell方程出发,利用多重尺度法,导出了光学整流场与两个基频电场包络的非线性耦合模方程组,给出了耦合模方程组的孤子解.结果表明,由于二阶非线性导致的光学整流场对基频电场有调制作用,使得两个基频电场分量可以呈现为亮孤子亮孤子、暗孤子暗孤子及亮孤子-暗孤子对 当两个基频电场的振动频率趋于光子晶体频带的带边频率时,光学整流场消失     

具有二阶和三阶非线性一维光子晶体中的耦合模孤子

研究同时具有二阶和三阶非线性的一维光子晶体中的耦合孤子动力学. 从Maxwell方程出发,利用多重尺度法,导出了光学整流场与两个基频电场包络的非线性耦合模方程组,给出了耦合模方程组的孤子解. 结果表明,由于二阶非线性导致的光学整流场对基频电场有调制作用,使得两个基频电场分量可以呈现为亮孤子-亮孤子、暗孤子-暗孤子及亮孤子-暗孤子对;当两个基频电场的振动频率趋于光子晶体频带的带边频率时,光学整流场消失.     

近场研究表面等离子体在银纳米线上的传输

通过双探针近场光学扫描显微镜在银纳米线上实现近场激发和近场收集表面等离子体,用一个探针在银纳米线的一端近场激发表面等离子体,另一个探针近场探测银纳米线上的表面等离子体强度分布,得到强度分布图.强度分布图显示表面等离子体在银纳米线的一端被有效激发并且有一部分表面等离子体沿着银纳米线和基底的界面传播到了另一端.用有限元法对银纳米线内的传播模式进行数值模拟,结果显示银纳米线内存在两种表面等离子体传播模式,分别为基模和高阶模.沿着银纳米线和基底介质之间传输的基模表面等离子体由于传输环境稳定,散射损耗小,实际传输长度接近模式传输长度,达10μm以上;而高阶模表面等离子体由于部分裸露在空气中受表面缺陷散射的影响,散射损耗大,实际传输长度远小于模式传输长度.研究表明:以能量高度束缚的基模表面等离子体作为载体,不仅可以实现低损耗传输,还可以减小集成器件之间的信号串扰,有效提高信息传输的安全性,在集成光学中具有重要应用.     

金纳米线与亚波长狭缝结合实现局域场增强研究

为了获得更强的局域场来增强喇曼散射信号或非线性效应,提出了在亚波长金狭缝中放置两列紧邻金纳米线的结构,将两纳米线近场耦合效应和狭缝类法布里-珀罗共振对电场的放大作用结合起来实现纳米线间更强的电磁场.理论分析得出,两纳米线间电场强度在狭缝深度增加时呈现周期性起伏变化,满足类法布里-珀罗共振条件时出现峰值,且纳米线对于发生共振时狭缝深度有调制作用;电场强度在狭缝周期近似等于入射波长附近呈现突变趋势,在纳米线间距增加时呈指数递减.用有限元法对增强机理进行了仿真,结果表明:在纳米线间距为1nm和2nm时,增强效果较好;狭缝周期为600nm、深度为310nm、宽度为100nm、纳米线间距为1nm,在波长650nm时,两金纳米线中心热点处电场增强为200倍,达到109的喇曼增强因子,比单纯的两根金纳米线的热点处增强因子提高了3个数量级.     

五边形截面的Ag纳米线局域表面等离子体共振模式

五边形截面的单晶Ag纳米线对ZnO量子点荧光具有增强的现象. 为解释这一现象, 利用时域有限差分法对五边形截面的Ag纳米线的局域表面等离子体共振模式进行了理论模拟. 结果表明, 五边形截面的Ag纳米线在紫外区域存在两个消光峰, 分别由Ag纳米线的横向偶极共振(340 nm)和四极共振(375 nm)引起; 这两个消光峰与ZnO量子点荧光增强峰相一致, 而且随着Ag纳米线的半径增大而红移; 消光峰对应的共振模式取决于Ag纳米线的截面形状; 根据Ag纳米线电场增强倍数与激发光波长变化关系曲线可知, 最大增强电场位于五边形截面的顶点处, 而边线处电场增强较小. 理论模拟的结果较好地解释了Ag纳米线/ZnO量子点体系的荧光增强现象, 也为Ag纳米线在提高半导体材料发光效率、生物探测等方面的应用提供有益的参考.     

Bright and dark spatial solitons in metallic nanowire arrays

We investigate the formation and propagation of bright and dark three-dimensional unstaggered spatial solitons with cylindrical symmetry in a nonlinear nanowire metamaterial. The metamaterial is formed by metallic nanowires embedded in a Kerr-type dielectric host and is modeled using an effective medium approach. Unlike conventional Kerr media, the metamaterial supports bright solitons when the host is a self-defocusing material and dark solitons when the host is a self-focusing material. Our numerical calculations show that the confinement of the spatial-solitons results from the interplay of the host nonlinear response strength and the hyperbolic dispersion of the photonic states in the nanowire array. Subwavelength solitary beams may be observed for sufficiently strong nonlinearities.     

Numerical study of guided modes in arrays of metallic nanowires

We numerically investigate the band structure and guided modes within arrays of metallic nanowires. We show that bandgaps appear for a range of array geometries and that these can be used to guide light in these structures. Values of attenuation as low as 1.7 dB/cm are predicted for arrays of silver wires at communications wavelengths. This is more than 100 times smaller than the attenuation of the surface plasmon polariton modes on a single silver nanowire.     

十字结构银纳米线的表面等离极化激元分束特性

金属纳米线波导可以将光局域在亚波长尺度内传播, 在纳米光子集成回路方面有着重要的作用. 本文应用有限元方法, 研究了十字结构银纳米线的表面等离极化激元分束特性. 结果表明, 不同模式的表面等离极化激元在十字结构三个分支的输出依赖于端面的几何结构参数. 此外, 研究还发现由于不同模式表面等离极化激元叠加, 在十字结构的分支上出现了周期性电场分布.     

紫外光辐照对TiO2纳米线电输运性能的影响及磁阻效应研究

采用溶胶凝胶法以及静电纺丝法, 利用热处理工艺, 成功制备出了多晶锐钛矿型TiO₂纳米线, 通过两线法在室温下测试单根TiO₂纳米线的V-I曲线来研究其电输运性能及磁阻效应. 结果表明: 在无光照环境下其V-I曲线为不过零点的直线, 零场电阻较大, 在磁场作用下电阻下降, 表现出负磁阻效应; 紫外光辐照环境下TiO₂纳米线载流子浓度增加使得电阻变小, 然而在磁场作用下电阻增大, 表现为正磁阻效应. 紫外光辐照导致的载流子浓度变化, 使得负磁阻转变为正磁阻, 我们将磁阻变化归结为d电子局域导致的负磁阻与能带劈裂导致的正磁阻两种机理相互竞争的结果.     

The effect of the electron-phonon coupling on the thermal conductivity of silicon nanowires

The thermal conductivity of free-standing silicon nanowires (SiNWs) with diameters from 1-3?nm has been studied by using the one-dimensional Boltzmann's transport equation. Our model explicitly accounts for the Umklapp scattering process and electron-phonon coupling effects in the calculation of the phonon scattering rates. The role of the electron-phonon coupling in the heat transport is relatively small for large silicon nanowires. It is found that the effect of the electron-phonon coupling on the thermal conduction is enhanced as the diameter of the silicon nanowires decreases. Electrons in the conduction band scatter low-energy phonons effectively where surface modes dominate, resulting in a smaller thermal conductivity. Neglecting the electron-phonon coupling leads to overestimation of the thermal transport for ultra-thin SiNWs. The detailed study of the phonon density of states from the surface atoms and central atoms shows a better understanding of the nontrivial size dependence of the heat transport in silicon nanowire.     

Nucleation of misfit dislocations and plastic deformation in core/shell nanowires

During fabrication of metal nanowires, an oxide layer (shell) that surrounds the metal (core) may form. Such an oxide-covered nanowire can be viewed as a cylindrical core/shell nanostructure, possessing a crystal lattice mismatch between the core and shell. Experimental evidence has shown that, in response to this mismatch, mechanical stresses induce plastic deformation in the shell and misfit dislocations nucleate at the core/shell interface. As a result, the mechanical, electrical and optoelectronic properties of the nanowire are affected. It is therefore essential to be able to predict the critical conditions at which misfit dislocation nucleation at the nanowire interface takes place and the critical applied load at which the interface begins deforming plastically. Two approaches are explored in order to analyze the stress relaxation processes in these oxide-covered nanowires: (i) energy considerations are carried out within a classical elasticity framework to predict the critical radii (of the core and shell) at which dislocation nucleation takes place at the nanowire interface; (ii) a strain gradient plasticity approach is applied to estimate the flow stress at which the interface will begin deforming plastically (this stress is termed “interfacial-yield” stress). The interfacial-yield stress, predicted by gradient plasticity, depends, among other material parameters, on the radii of the core and shell. Both approaches demonstrate how the geometric parameters of nanowires can be calibrated so as to avoid undesirable plastic deformation; in particular, method (i) can give the radii values that prevent misfit dislocation formation, whereas method (ii) can provide, for particular radii values, the critical stress at which interface deformation initiates.     

Surface effect on free transverse vibrations and dynamic instability of current-carrying nanowires in the presence of a longitudinal magnetic field

Free transverse vibration and instability of current-carrying nanowires immersed in a longitudinal magnetic field are of concern. On the basis of the surface elasticity theory, a model is developed to investigate the problem. The analytical expressions of dynamic transverse displacements as well as natural frequencies of the magnetically affected nanowire for carrying electric current are obtained. The influences of the surface effect, initial tensile force within the nanowire, strength of the longitudinal magnetic field, and electric current on the natural frequencies as well as dynamic displacements are examined. The obtained results reveal that the transverse stiffness of the nanostructure is enhanced by the surface effect and the initial tensile force, while electric current or longitudinal magnetic field reduces the nanowire's stiffness. The condition which leads to the dynamic instability of the nanostructure is obtained. Further, the roles of the influential parameters on its stability are inclusively discussed.     

锡催化剂助生长法制备孪晶ZnO纳米线及其结构表征

用SnO和Zn的均匀混合物在高温下共烧通过VLS机制制备出孪晶ZnO纳米线的均匀结构。SEM图像显示孪晶ZnO纳米线的直径在100~200nm之间,长度在几十微米到几百微米之间的范围内,有的甚至达到了毫米级,产率也非常的高。TEM图像中ZnO孪晶纳米线顶端的金属Sn颗粒表明了孪晶结构的Sn催化生长。高分辨电子图谱显示了氧化锌纳米线孪晶结构的特征。电子衍射分析发现孪晶氧化锌的晶带轴的方向是[0110],孪晶面为(1013),并且通过明场像和暗场像分析了孪晶纳米线的晶格关系,确定了孪晶纳米线的汽-液-固(VLS)生长机制。     

The stability and electronic properties of wurtzite and zinc-blende ZnS nanowires

The stability and electronic properties of the pristine wurtzite (WZ) and zinc-blende (ZB) structural ZnS nanowires are investigated and compared by using first-principles approaches. It shows that the WZ-ZnS nanowire is more stable energetically than the ZB-ZnS nanowire. The two kinds of ZnS nanowires have different electronic properties due to both the quantum confinement effect and the surface effect. The band gaps of pristine WZ nanowires become larger than that of the corresponding bulk ZnS, while those of ZB nanowires are smaller. The electronic properties of the hydrogen-passivated WZ-ZnS and ZB-ZnS nanowires are further calculated. The underlying physical reason for their energetic and electronic structures is elucidated.